Ghost galaxies may haunt the Milky Way

Though telescopes are capable of detecting galaxies billions of light years distant, they may be missing many in our own cosmic backyard. Hundreds of our nearest neighbours could have eluded detection due to their feeble light output, new calculations suggest.

Telescopic surveys have detected a handful of very faint nearby galaxies, the dimmest of which is just a few hundred times as bright as the sun.

But even dimmer galaxies are likely hiding out there, say James Bullock of the University of California in Irvine and his colleagues (arxiv.org/abs/0912.1873).

Galaxies with less dark matter than those found to date would have a weaker gravitational hold on their stars, allowing them to spread out more. These more diffuse galaxies would be especially hard to spot amidst the clutter of foreground stars in our own galaxy.

“There absolutely could be a population of extremely low-surface-brightness dwarfs that have evaded detection,” says Beth Willman of Haverford College in Pennsylvania, who was not involved in the study.

Future observatories, such as the Large Synoptic Survey Telescope could find them, she says.